1. Field of the Invention
This invention relates to a sheet feeder, and more particularly to a sheet feeder including a feed roller for feeding paper.
2. Description of the Related Art
Currently, there are various types of known sheet feeders equipped with feeding rollers for feeding sheets of paper (for example, see JP-A-61-197349, JP-UM-A-6-64981 and JP-A-10-139235).
JP-A-61-197349 discloses a sheet feeder in which hindrance of paper feeding caused by the tip of curled paper impinging on a pair of paper feeding rollers is avoided by means of an auxiliary ring which smoothly introduces the tip of paper into the contacting portion between the feeding rollers.
JP-UM-A-6-64981 shows another sheet feeder in which backlash by a feed roller against a drive shaft during rotation is eliminated by bringing substantially D-shaped mating holes in fixing members attached to both sides of the feed roller into engagement with the drive shaft having a corresponding D-shaped cross-section.
JP-A-10-139235 describes still another sheet feeder in which the number of components is reduced by forming an acute-angled elastic rib for pressing a discharge roller against a feed roller to be integral with the discharge roller, as compared with a sheet feeder which includes a separate member for pressing the discharge roller.
FIG. 7 is a front view illustrating a conventional sheet feeder including a feed roller as an example. FIG. 8 is a cross-sectional view of the sheet feeder taken along a line 150—150 in FIG. 7. FIG. 9 is another cross-sectional view showing the sheet feeder taken along a line 160—160 in FIG. 7. Referring to the structure of the conventional sheet feeder in FIG. 7, the sheet feeder contains a feed roller 101 for feeding sheets of paper 200. The feed roller 101 has a rotational shaft 101a. The rotational shaft 101a of the feed roller 101 is inserted into bearings 102 provided adjacent to both ends of the rotational shaft 101a leaving a predetermined width of play between the bearings 102 and the rotational shaft 101a. As illustrated in FIG. 8, each of the bearings 102 has a circular inner surface 102a having an inside diameter larger than the diameter of the rotational shaft 101a of the feed roller 101. The rotational shaft 101a of the feed roller 101 is rotationally supported on the bearings 102 in such a manner that the surface of the rotational shaft 101a is held on the inner surfaces 102a of the bearings 102.
A gear (not shown) for transmitting driving force from a drive motor (not shown) is equipped at one end of the rotational shaft 101a of the feed roller 101. As illustrated in FIG. 7, pinch rollers 103 are provided for pressing the paper 200 against the feed roller 101. The pinch rollers 103 are attached to a rotational shaft 103a. The rotational shaft 103a of the pinch rollers 103 is rotationally supported by a holder 104. The holder 104 presses the pinch rollers 103 against the feed roller 101 by means of a spring (not shown).
An operation of the related-art sheet feeder as shown in FIG. 7 is now described referring to FIGS. 8 and 9. In operation of the sheet feeder in FIG. 7, the feed roller 101 is rotated in a direction shown by an arrow A in FIG. 9 by the driving force transmitted from the drive motor (not shown) through the gear (not shown) to the rotational shaft 101a of the feed roller 101. During rotation of the feed roller 101, the rotational shaft 101a of the feed roller 101 is rotated while contacting the inner surface 102a of the bearing 102 at a contacting point P which is inclined from the lowermost position through an angle of θ1 as illustrated in FIG. 8. The pinch rollers 103 are then rotated in a direction of an arrow B in FIG. 9 along with the rotation of the feed roller 101. Thus, the paper 200 inserted between the feed roller 101 and the pinch rollers 103 is conveyed in a direction of an arrow C in FIG. 9. At this stage, the paper 200 applies a load W to the feed roller 101 in a direction opposite to the moving direction of the paper 200 (i.e., the direction of the arrow C in FIG. 9).
According to the conventional sheet feeder shown in FIG. 7, however, a problem has been arising that the position of the contact point P between the rotational shaft 101a of the feed roller 101 and the inner surface 102a of the bearing 102 varies due to fluctuation in the load W (shown in FIG. 8) applied to the feed roller 101 from the paper 200. More specifically, the contact point P moves in such a direction that the angle θ1 becomes smaller when the load W is larger, while the contact point P moves in such a direction that the angle θ1 becomes larger when the load W is smaller. Thus, positional variation of the feed roller 101 is caused in a direction parallel to the feeding direction of the paper 200 (i.e., the direction of the arrow C in FIG. 9), which changes the feeding volume of the paper 200 by the operation of the feed roller 101. Accordingly, the sheet feeder of FIG. 7 has a drawback that the accurate control of the feeding volume of paper is difficult.
Similarly, in the sheet feeders disclosed in JP-A-61-197349, JP-UM-A-6-64981 and JP-A-10-139235, it is difficult to accurately control the feeding volume of paper as in the sheet feeder shown in FIG. 7 since no measure is taken for preventing the rotational shaft of the feed roller from shifting with respect to the bearings.